Porous Silicon (hereinafter referred to as PS) is a substance in which infinite numbers of pores in nanometers are formed in silicon. The porous silicon is formed by anodic oxidation (hereinafter referred to as forming) of silicon in an electrolyte solution containing hydrogen fluoride (HF) acid. As to the porous silicon of this kind, its fundamental forming method and physical properties have been studied as a material which can be expected to be applied to various products.
Especially, as shown in FIG. 19A, in a method of whole surface forming in which the whole of one surface 3 of a silicon substrate 1 is anodically formed, HF acid concentration and forming current are uniformly distributed from the whole surface of the substrate. Even if the forming proceeds to an arbitrary thickness (d) of a PS layer, an interface area between porous silicon 120 (FIG. 19A) and the silicon substrate 1 is constant at all times. Consequently, forming conditions do not drastically change with forming depth (d). That is, even if the forming is continued at constant forming current 121 (FIG. 19B), HF acid concentration and PS/Si interface current density (I.D.C.) 122 (FIG. 19B) can be easily kept at constant. Accordingly, this method makes it possible to obtain porosity 131 (FIG. 19C) and pore size 132 (FIG. 19C) independent on the forming depth (d), and there have been reported a number of studies about it as an optimum method for studying a relationship between a porous silicon forming method and physical properties of the porous silicon. Among these studies, [1] R. Herino, G. Bomchil, K. Barla, C. Bertrand, and J. L. Ginoux, J.Electrochem. Soc. 134, 1994(1987) states details of a relationship between forming conditions and physical properties of the porous silicon to be formed in the whole surface forming.
As to a method of depositing a mask layer on a silicon substrate, and anodizing the silicon substrate centering around an opening area of said mask layer so as to selectively form the porous silicon region (hereinafter this method is referred to as selective forming), there are following major documents [2]-[5]: [2] P. Steiner and W. Lang, Thin Sold Films 255, 52 (1995), [3] K. Imai and H. Unno, IEEE Trans. Electron Devices ED31, 297 (1984), [4] V. P. Bondarenko, V. S. Varichenko, A. M. Dorofeev, N. M. Kazyuchits, V. A. Labunov, and V. F.Stel'makh, Tech. Phys. Lett. 19, 463 (1993), [5] V. P. Bondarenko, A. M. Dorofeev, and N. M. Kazuchits, Microelectronic Engineering 28, 447 (1995) and so on. The above documents [2]-[5] introduce a method of the forming in a condition that the forming current is kept at constant.
The document [4] indicates that selectively formed PS is oxidized and employed for an optical waveguide. However, it does not give a detail explanation about factors of change in a refractive index for confining light in the waveguide. Further, the document [5] indicates that an impurity can be doped into the selectively formed PS. However, it does not disclose a thought that an impurity is selectively doped into a specific PS region.
On the other hand, as to a known document for changing forming current with time in PS forming, [6] M. Berger et al. PCT/DE96/00913 discloses a method for it. Also, [7] A. Loni, L. T. Canham, M. G. Berger, R. Arens-Fischer, H. Munder, H. Luth, H. F. Arrand, and T. M. Benson, Thin Solid Film 279, 143 (1996) discloses a method for steeply changing continuous direct current in stages, thereby forming porous silicon in which porosity intermittently changes depending on current value.
The above prior arts [6] and [7] mainly disclose that the multilayer porous silicon having high porosity (60% or more) is formed and grown into an optical waveguide itself by employing the property that the refractive index of the porous silicon depends on the porosity, and disclose that the porous silicon is oxidized and the silicon dioxide which is not densified in a porous state, is grown into an optical waveguide. In the documents [6] and [7], forming current is changed with time and in stages, and kept at constant for a predetermined period.
As to documents mainly aiming at forming by pulse current, the following documents are given: [8] Xiao-yuan Hou, Hong-lei Fan, Lei Xu, Fu-long Zhang, Min-quan Li, Ming-ren Yu, and Xun Wang, Appl. Phys. Lett., 68, 2323 (1996), and [9] L. V. Belyakov, D. N. Goryachev, and O. M. Sreseli, Tech. Phys. Lett. 22, 97 (1996). Both of the documents [8] and [9] compare an effect of the pulse current with that of continuous direct current in the whole surface forming of the silicon substrate. However, they disclose nothing about the selective forming.
Next, as to documents about oxidation of the porous silicon, there are following documents: [10] J. J. Yon, K. Balra, R. Herino, and G. Bomchil, J. Appl. Phys., 62, 1042 (1987), and [11] K. Balra, R. Herino, and G. Bomchil, J. Appl. Phys., 59, 439 (1986).
The above technical literatures [10] and [11] concern oxidation of porous silicon of the whole surface forming, but do not concern selectively formed porous silicon. They emphasize the importance of porosity control because volume expansion and shrinkage caused by oxidation considerably depend on porosity. However, these documents state that the volume expansion caused by oxidation affects increase of thickness of a silicon dioxide film in the whole surface forming even if the porosity is equal to or lower than the later-explained critical porosity.
All of the above-mentioned prior arts [1]-[11] do not disclose a thought for designing porosity of the selectively formed porous silicon to be constant. The selective forming has following problems.
As shown in FIG. 17 for the later-described compared example, when the selective forming is carried out in the silicon substrate 1 (FIG. 17A) on which the mask layer 3 having an opening area 7 is deposited, an interface area 111 (FIG. 17B) between the porous silicon 100 and silicon 1 changes (increases) as the forming proceeds. If the forming is carried out at constant forming current, interface current density 113 (FIG. 17C) in an interface 102 between porous silicon 100 and silicon 1 (FIG. 17D) relatively decreases as the forming proceeds. Accordingly, both of porosity 115 (FIG. 17D) and pore size of the porous silicon also decrease as the forming proceeds, wherein there is a problem that these values cannot be kept at constant. Further, in the selective forming, a lot of problems are caused by a limited condition that a supply route for HF in the forming solution, a supply route for forming current, and an escape route for anodic gas generated in the forming concentrate into the part 7 (FIG. 17A) from which said mask layer is removed.
In addition, various problems are caused by the volume change after oxidizing the porous silicon in the selective forming. Low porosity expands the volume after oxidation, and partially brings about inside stress on the PS/Si interface, which reduces the reliability of the applied devices. However, high porosity extremely shrink the volume after oxidation. If the porous silicon is applied to an optical waveguide and so on, the volume shrinkage makes it difficult to form the waveguide having a desirable shape. This invention makes it possible to minimize the volume expansion and shrinkage of the porous silicon after the oxidation by porosity control of the selectively-formed porous silicon, and besides, allows the selective doping of an impurity by pore size control.